The invention relates to a process and a device for wetting wood fibres with a binding fluid, in particular for dry sizing of wood fibres. The invention also relates to a process for manufacturing a fibre board as well as the fibre board itself.
In general terms, the invention relates to applying a fluid to solid particles in a conveying air stream.
The manufacture of fibre boards such as e.g. medium-density fibre board (MDF), high-density fibre board (HDF) and fibre boards of low density (LDF) according to the dry method is known. Lumpy wood is pulped in the pulper by the effect of pressure and temperature in a saturated steam atmosphere. The lumpy wood thus softened reaches the refiner, in which it is mechanically pulped into fine wood fibres.
A pipe, the so-called blowline, guides the mixture of steam, water and fibres from the refiner to the dryer. In the blowline the fibres travel at a very high speed in the vicinity of 30 to 100 m/sec. The sudden drop in pressure when the water vapour-water-fibre mixture exits from the blowline and enters the dryer supports singling out the fibres. Fibre agglomerates can be shredded, so that subsequent drying in the bus tube dryer brings the fibres effectively in a few seconds to fibre humidity by ca. 10%, relative to the dry mass.
Cyclones separate the dried fibres from the air flow and via conveyor equipment these are fed to a sifter for separating out glue lumps, fibre agglomerates or other entrained lumps, which detach from the inner wall of the bus tube dryer and/or from the lines. The dried fibre material thus treated reaches the former, where a fibre cake of minimal thickness (20 to 30 kg/m3) is formed. Under the effect of pressure and temperature a board is formed in a press, which may have a thickness of 2 to 50 mm and a density between 60 to 1000 kg/m3.
The above described manufacturing technology known from the prior art provides for supplying the binding agent to the mixture of water and wood fibres in the blowline, and also on the path of the fibres between refiner output and dryer input. The binding agent is thus subjected to a high temperature of well over 100° C. for a certain period from being fed to the fibres. This is significant insofar as the binding agent is to be cured in the press by the action of temperature. Usual binding agents are condensation resins such as aminoplasts (urea formaldehyde resin (UF), melamine formaldehyde resin (MUF) or mixtures thereof) and/or isocyanates (e.g. PMDI). The reaction capacity of the resins must match the increased temperature requirements during gluing and drying insofar as the latter react very sluggishly. This is reflected in the curing rate. If the press factor (dwell time of the board in seconds per millimetre of board thickness in the press) is compared, then that of a MDF board is in the region by 8 to 12 s/mm of that of a particle board of comparable density and same thickness by 4 s/mm. Therefore a board press of the same size for particle board has a performance higher by ca. 50% than that for MDF. In addition, the high press factor for MDF is also influenced by other parameters such as e.g. heat penetration, steam transport from the exterior to the board centre, steaming out on the press end. The essential influence is however the sluggish reactivity of the binding agent.
Acceleration testing with e.g. hardeners or another production method for resins have so far not shown any success, since the associated advanced curing in the dryer has not brought about any improvement of mechanical board properties or any reduction in the press factor and/or any reduction in the required quantity of adhesive.
Also, the binding agent in the blowline is subjected to water, so that the binding agents are also curtailed in this respect. Different binding agents, which are suitable per se for producing fibre boards, cannot be used for contact with water, or can be used but only limited. This applies in particular for isocyanates. So-called encapsulated isocyanates are in use, and are suited principally for a blowline adhesion, yet trouble-free operation over several days is not possible. As a rule the blowline accrues through isocyanate reacting with water and the plant must be shut down for cleaning.
The water present in the blowline has a minimal pH value, which results from the previous cooking of the wood chips. Aminoplasts such as urea formaldehyde resins (UF) and melamine formaldehyde resins (MF) are acid hardening, which is why advanced hardening already takes place in the blowline.
The technical problem of the present invention is now to improve the wetting of wood fibres with a binding agent.